The present invention relates to the use of free-radically curable compositions, comprising chain-like and/or cyclic and/or cage-type polysiloxanes substituted by free-radically polymerizable groups and having at least 3 silicon atoms and/or mixed forms thereof, disiloxanes substituted by free-radically polymerizable groups, optionally one, two, three or more free-radically curable monomers having no silicon atom, fillers, initiators and/or catalysts for free-radical polymerization, and also further customary additives, in additive manufacturing methods, preferably in stereolithography (SL) and digital light processing (DLP). The present invention further relates to the use of the cured free-radically curable compositions, preferably for production of dental products.
Further aspects of the present invention and preferred configurations thereof will become apparent from the description which follows, the working examples and the claims.
The term “additive manufacturing method” is an established expression (see A. Gebhard, “Generative Fertigungsverfahren”, 4th edition, Carl Hanser Verlag, Munich 2013) and is also known to those skilled in the art as “rapid prototyping” (RP). It describes, proceeding from a CAD dataset as a 3D geometry model, the layer-by-layer, tool-free formation of a particular shaped body.
The most commonly used RP method nowadays is stereolithography (see U.S. Pat. No. 4,575,330). This method works with an ultraviolet laser which cures suitable resin systems layer by layer. In general, conventional (meth)acrylate resins which have been provided with suitable photoinitiators are polymerized. A vertically movable platform having a suitable surface (metal, glass, ceramic, etc.) for the solid resin phase to grow onto is lowered to the distance of one layer thickness (20-50 μm), measured from the base of the liquid resin bath. Then there is localized crosslinking and curing of the resin by a laser, which is guided into the resin bath, for example, from beneath by means of micro-mirrors. The solid polymer that forms on the platform, after the crosslinking of a first resin layer, is moved up by one further layer thickness in the resin bath and hence covered again with liquid resin. The laser now cures the second layer. The sequence of raising the movable platform furnished with resin layers and the subsequent polymerization are repeated until the entire 3D model has been formed.
It is a characteristic feature of RP methods that the polymerizable groups are not completely converted during the layer-by-layer formation of the shaped body, and so the resulting parts still do not have sufficient final hardness and strength. The objects produced therefore have to be subjected to further curing in a further step of thermal aftertreatment in an oven or subsequent irradiation in a light box.
DLP is a modification of SL. Rather than a laser, in this method, visible non-coherent light is used for the polymerization of the curable resin. This methodology uses a projected image for selective polymerization of the resin. The pattern required is imaged directly onto the resin by a dynamic LCD mask, such that the entire layout is polymerized simultaneously, with curing of the exposed sites in the layer, leaving the unexposed sites uncured. Compared to curing by laser, which is effected only at particular points, this method is therefore much faster than SL.
Further RP methods are polyjet technology, the galvanometer-type scanning method, micro-stereolithography, multijet modeling, selective laser sintering, 3D printing, fused deposition modeling, 3D plotting, laminated object manufacturing or film transfer imaging.
Because of the versatility of the RP methods and the advantage of these methods compared to material-removing manufacturing methods, for example in terms of the more efficient production and the almost unlimited geometric freedom of the components and moldings, RP is becoming established in entire fields of industry, such as jewelry, architecture, design, medical technology, and here especially also in dentistry, where absolutely correct impression and duplicating methods are of course important.
For example, in dentistry, the procedure begins with the digital recording of the situation in the mouth by the 3D scan of a prepared cavity, in order thus to manufacture a dental molding on the basis of CAD (computer-aided design) data. The model generated is subsequently produced by using a shaping method, for example stereolithography, layer by layer from a liquid or pasty resin. Dental shaped bodies, such as inlays, onlays, veneers, crowns, bridges, artificial teeth, dentures, scaffolds, temporary prostheses and orthodontic products, can be manufactured directly in the dental practice (or in the dental laboratory) without any great effort or material loss (as occurs in the machining of blanks). The methods proceed in a simple and rapid manner, and they can even simultaneously manufacture several moldings/components from different orders.
Additive manufacturing methods, specifically including stereolithography methods, for production of dental shaped bodies are known from the prior art.
DE 697 04 623 T2 specifies a method for producing a three-dimensional article from a curable liquid medium, wherein the article is formed layer by layer, by each time applying a layer of liquid medium to a carrier and/or an already formed part of the article in a vessel containing liquid medium and then curing said layer.
WO 2013/153183 A2 describes composite resin compositions and methods for production of dental components by means of stereolithography. What is claimed is the use of a dental composition comprising a polyreactive binder, two photopolymerization initiators having different absorption maxima and an absorber.
DE 199 38 463 A1 discloses compositions curable with visible light, containing 2%-99% by weight of a curable resin, 0.01%-7% by weight of an initiator, 0%-5% by weight of a coinitiator and 0%-85% by weight of one or more modifiers such as fillers, dyes, pigments, flow improvers, thixotropic agents, polymeric thickeners, oxidizing additives, stabilizers and retardants for use in a shaping method. Methods are specified for microconsolidation, RP, film casting, the production of sintered polymer components, microstructuring, photolithography, the production of dental products, the production of surgical implants and/or the production of otoplastic products. The application document describes and depicts the structure of a stereolithography apparatus. In the inventive examples, filler-free free-radically curable compositions based on a mixture of butane-1,4-diol dimethacrylate, aliphatic diurethane methacrylate and aliphatic urethane methacrylate, and a mixture of butane-1,4-diol dimethacrylate, aliphatic diurethane methacrylate, aliphatic urethane methacrylate and tetraethoxylated bisphenol A dimethacrylate are used.
DE 199 50 284 A1 is for the most part identical to the above-cited DE 199 38 463 A1. However, in this application document, additional free-radically curable systems from the publications DE 41 33 494 C2 and DE 39 03 407 A1, based on polysiloxanes, are integrated into the application text. However, this application document does not contain inventive compositions comprising polysiloxanes in form of examples.
DE 101 14 290 B4 is aimed at 3D plotting (an alternative method to stereolithography) for production of dental moldings, wherein curable resins from nozzles are applied to a suitable construction platform. This document also mentions silicone resins which lead to polymer networks through condensation or hydrolysis as resin systems.
DE 10 2012 012 346 A1 relates to shaped bodies made from dental material that remains soft, especially a gingival mask, and methods for production thereof by means of RP. The shaped body made from dental material that remains soft, especially the gingival mask or relining for a dental prosthesis, is said to be curable layer by layer to give an elastomer by means of an RP method involving light curing of a radiation-curable composition, especially by means of UV-A and/or UV-B rays. With particular preference, it is said to be possible to use compositions comprising radiation-curable silicones which may contain further auxiliaries, fillers, pigments or thinners. In this context, silicones functionalized with at least two alkene groups in a hydrosilylation reaction and hydridic silicones having at least two Si—H functionalities can be converted in the presence of a hydrosilylation catalyst. Alternatively, it is also said to be possible to use silanol-terminated polysiloxanes and silane crosslinking agents selected from vinyltrimethoxysilane, vinyltriaminosilane, vinyltriamidosilane, vinyltrioximosilane, vinyltriiso-propenoxysilane or vinyltriacetoxysilane and a photoinitiator.
The silanes specified in DE 199 34 407 A1 are said to have low viscosity and be flexible, which can be processed alone or together with other hydrolyzable, condensable and/or polymerizable components to give silicic acid polycondensates or to give silicic acid heteropolycondensates, the ultimate curing of which is then to be effected by polymerization of the C═C double bonds. The hydrolyzable and condensable silanes according to the invention are intended for use in specific applications, for example the coating of substrates of metal, plastic, paper, ceramic (by dipping, casting, painting, spraying, electrostatic spraying, electrocoating), for production of optical, optoelectronic and electronic components, for production of fillers, for production of scratch-resistant, abrasion-resistant anticorrosion coatings of shaped bodies, for example by injection molding, casting, pressing, rapid prototyping or extrusion, or for production of composites, for example with fibers, fillers or woven fabrics. As well as the fields of use of optics, electronics, medicine, especially dentistry and optoelectronics, the field of food packaging is also mentioned.
As a result of the method operations, the resin systems from the prior art used in additive manufacturing methods, for example in stereolithography, lack the desired precision in the geometric configuration of the shaped bodies. This is because the use of a radiation source in the layer-by-layer polymerization of the resin, through scattered and/or deflected photons, also results in concomitant curing of regions outside the defined shape. In order to minimize dimensional inaccuracy, what are called “absorbers” are generally added to the free-radically curable compositions. These are molecules which absorb radiation. In the literature, these compounds are also known as stabilizers or inhibitors. In general, benzotriazoles, triazines, benzophenones are used. Mention should also be made of salicylic acid derivatives and the hindered amine light stabilizers (HALS). Inorganic salts such as nanoscale titanium dioxide or zinc oxides can also assume a radiation-absorbing function. Their use can greatly improve dimensional accuracy, but this addition also reduces the through-curing depth and the conversion during free-radical curing.
It was therefore an object of the invention to provide a resin system having much higher dimensional accuracy in an additive manufacturing method, for example stereolithography. In addition, the resin system should also exhibit improved biocompatibility compared to conventional binders. Furthermore, the resin systems of the invention should have very good wettability on different substrates, a high conversion rate, a low water absorption, good mechanical strength and exceptionally low shrinkage.